Radiative and non-radiative processes in nearly strain-free AlxGa1–xN alloys were studied by means of steady-state and time-resolved photoluminescence spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and photoluminescence measurements gave a bowing parameter b of about –0.82eV. Values of the full-width at half-maximum of the near-bandedge photoluminescence peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts were as large as 0.10 to 0.25eV and both Stokes-type shifts and full-width at half-maximum of the photoluminescence increased with the increase in x for x less than 0.7. Simultaneously, the luminescence red-shift due to the increase in temperature T from 8 to 300K decreased with increasing x and approached zero for x = 0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The time-resolved photoluminescence signals exhibited a bi-exponential decay at low temperatures, and the slower component became longer with increasing x (over 40ns for x = 0.49). Simultaneously, density or size of cation vacancies (VIII) and relative intensity of the deep-level emission over that of the near-bandedge one at 300K increased as x increased to 0.7. Consequently, certain trapping mechanisms associated with VIII where suggested, and excitons were then de-trapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire time-resolved photoluminescence signals signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the time-resolved photoluminescence signals signal became single exponential due to the increasing dominance of non-radiative recombination processes in the free states, resulting in lower internal quantum efficiency with increasing x for x less than 0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters required further reduction of the non-radiative defect density as well as the VIII-related trap density.

Radiative and Non-Radiative Processes in Strain-Free AlxGa1–xN Films Studied by Time-Resolved Photoluminescence and Positron Annihilation Techniques. T.Onuma, S.F.Chichibu, A.Uedono, T.Sota, P.Cantu, T.M.Katona, J.F.Keading, S.Keller, U.K.Mishra, S.Nakamura, S.P.DenBaars: Journal of Applied Physics, 2004, 95[5], 2495-504